Conventionally, a NOx catalyst of a NOx storage-reduction type which stores (occludes) NOx within exhaust gas when an air-fuel ratio of the exhaust gas is lean, that is, larger than a stoichiometric air-fuel ratio (i.e., an air excess ratio λ>1) and reduces the stored NOx when the air-fuel ratio of the exhaust gas is close to the stoichiometric air-fuel ratio (λ≈1) or rich, that is, smaller than the stoichiometric air-fuel ratio (λ<1), and a PM filter which captures particulate matter are provided in an exhaust passage of the engine. Moreover, a control for maintaining a high purifying performance of these devices is executed.
For example, JP2016-109041A discloses an engine having an oxidation catalyst which oxidizes HC and CO, a NOx catalyst of a NOx storage-reduction type, and a PM filter which captures particulate matter. JP2016-109041A also discloses a configuration in which a control for injecting fuel into a cylinder from a late stage of expansion stroke to exhaust stroke to supply unburned fuel as a reducing agent to the NOx catalyst in order to reduce NOx stored in the NOx catalyst is executed. A configuration is also disclosed in which the fuel is injected into the cylinder from the late stage of the expansion stroke to the exhaust stroke to cause an oxidation reaction of the unburned fuel by the oxidation catalyst so as to raise temperature of the exhaust gas. Thus, combustion of the particulate matter captured by the PM filter is stimulated in order to maintain a high purifying performance of the PM filter.
In engines provided, for example, in a vehicle, improvements in exhaust performance and fuel efficiency are desired. However, in order to reduce the NOx stored in the NOx catalyst and combust to remove the particulate matter captured by the PM filter as described above, the fuel which does not contribute to an engine torque needs to be supplied to the cylinder, etc., and a difficulty in sufficiently improving the fuel efficiency arises as an issue.